Quadrupole residual gas sensors are known and are used for detecting the presence and measuring the quantity of specific gases within a chamber in vacuum conditions, e.g., at pressures of 1×10−5 Torr or below. A conventional quadrupole residual gas sensor includes four parallel rods, with equal lengths, precisely arranged and mounted on a ceramic base in a square configuration, thus forming a quadrupole, with an open area, or channel, at the center and extending the full length, of the rods. An electron source generates electrons at one end of the quadrupole which collide with, and ionize, some of the ambient gas molecules in the chamber. Some of these ions are then injected into the channel for mass analysis prior to reaching a collector positioned at the other end of the quadrupole. The ions that impact upon the collector generate a voltage potential upon the collector proportional to the number of ions and thus proportional to the population of gas molecules within the chamber. When the collector is connected to external circuitry, a current, proportional to the amount of ions impacting upon the collector is thereby generated.
Within the channel, ions are filtered according to their mass-to-charge ratio by applying a combination of AC and DC voltages of the same polarity to diagonally opposed rods. AC and DC voltages of the same amplitude but of the opposite polarity are applied to the other diagonally opposed rods. Voltages, applied on the four parallel rods of the quadrupole, are tuned to generate an electric field in the channel between the four rods which permits only ions with a specific mass-to-charge ratio to trail the full length of the channel down to the collector. Ions with other mass-to-charge ratios are pulled by the electric field from the channel to one of the four parallel rods and neutralized. Hence, by tuning the voltages on the rods for different mass-to-charge ratios, and by analyzing the current generated by ions impacting on the collector at these voltages, the quadrupole can be used to detect the presence of different gases within a chamber under low pressure or vacuum conditions. The ability to sense these gases is useful for a wide range of applications including planetary exploration, semiconductor chip manufacturing, industrial processing, environmental monitoring, petrochemical and shale gas applications.
In many applications where there is a need to determine what gases exist in a chamber, the pressure in the chamber is substantially higher than the pressures necessary to operate the prior art sensor. For example, in the thin film deposition techniques used in the manufacture of semiconductor devices, the films are often deposited in chambers where the pressure may even be two orders of magnitude greater than the pressure needed to operate the above-described prior art sensors. However, to maximize the sensitivity of the sensor, length the ions must travel in the channel to the collector must be less than the mean free path of the ions. The mean free path of an ion is the mean distance the ion will travel in a straight line through its environment prior to colliding with another molecular particle. The channel length must, preferably, be less than the mean free path of the particle to thereby minimize the likelihood of an ion, with the tuned mass-to-charge ratio, colliding with another particle and being deflected out of the channel or neutralized. Tuned ions which are deflected in this manner will not impact upon the collector, resulting in a lower current being detected at the collector. The mean free path of a particle, such as an ion, can be calculated by a well known formula in which the mean free path is inversely proportional to the ambient pressure of the environment that the particle is in. Hence, conventional four rod (each rod is approximately 15 cm long and 1 cm in diameter) quadrupole residual gas sensors operate at relatively low pressures, e.g., 5×10−5 Torr, to be able to obtain a mean free path greater than the length of the channel between the ion source and the collector.
One way to provide a quadrupole filter that allows operation at higher pressure is to miniaturize components of the filter to shorten the distance the ions travel. Any miniaturization effort results in an unavoidable reduction of instrument sensitivity due to the smaller acceptance area for the ions to be mass analyzed. Assembling arrays of quadrupoles to operate in parallel is one way to recovered some of the lost sensitivity due to miniaturization. Although this was suggested several decades ago, arrays of quadrupoles were difficult to assemble using conventional fabrication techniques. This has been addressed using a low cost glass-to-metal seal technology whereby the rods are held a glass chassis and voltages of opposite polarity are connected to alternating rods using thin photo-etched plates, as described in U.S. Pat. No. 5,613,294 to Ferran. The patent discloses a gas sensor having an array of quadrupoles (nine of them) formed by positioning a 4×4 array of rods (averaging 15 mm in length and 1 mm in diameter); a total of 16 in a matrix-like pattern.
The quadrupole array disclosed by Ferran uses a Faraday cup to detect ions of interest that reach the end of the chamber containing the rods. While detection with a Faraday cup is useful in many situations, in some cases it may be more desirable to use a different, more sensitive, detector, such as an electron multiplier. Electron multiplication based detectors such an electron multiplier and micro-channel plates contain a highly resistive coating on their internal surface. Incoming charged particles (e.g. ions and electrons) photons (e.g. UV and x-rays) release electrons upon colliding with the surface, via a secondary emission process, which triggers an avalanche process releasing more and more electrons. These electrons are channeled toward the exit via a high voltage bias between the entrance and the exit of the detector. The electron-to-ion conversion also called gain can achieve several orders of magnitude depending on the applied voltage.
However, because of miniaturization and the plurality of rods attached to the end of the sensor chassis, the quadrupole array disclosed by Ferran does not accommodate an electron multiplier.
Accordingly, it is desirable to provide a miniature quadrupole array like that taught by Ferran that can use an electron multiplier to detect gas ions of interest, possibly at much lower concentrations.